1. Field of the Invention
The present invention relates to a novel active energy ray curable aqueous ink composition exhibiting excellent curing performance by including a water-soluble polymerizable substance having high reactivity which undergoes radical polymerization by active energy rays and a water-soluble polymerization initiator having high initiation efficiency which generates radicals by active energy rays, and to an inkjet recording method, ink cartridge, recording unit, and inkjet recording apparatus using the active energy ray curable aqueous ink composition.
2. Related Background Art
Active energy ray curable aqueous ink can be preferably used for image printing, pretreatment for providing printability to a recording medium, aftertreatment for protection or decoration of the printed image, and the like. Further, the active energy ray curable aqueous ink technology has numerous excellent features and possibilities in that the ink can keep up with tightening of regulations on. VOC, can be applied to a high density inkjet recording head through reduction of ink viscosity, etc.
A polymerizable substance which undergoes radical polymerization by active energy rays (hereinafter, referred to as polymerizable substance) and a polymerization initiator which generates radicals by active energy rays (hereinafter, referred to as polymerization initiator), which are basic components of the active energy ray curable aqueous ink, each had been generally prepared in a form of emulsion in a water-based solvent initially. However, the polymerizable substance and the polymerization initiator themselves must be water-soluble or hydrophilic to prepare homogenous ink from the viewpoints of film formation properties of the ink during polymerization, discharge stability thereof applied to an inkjet system, and the like.
A first example of a water-soluble or hydrophilic polymerizable substance in practical use is a compound having an acidic group, and a (meth)acryloyl group or vinyl group in a molecule such as an ester of succinic anhydride and 2-hydroxyethyl(meth)acrylate, an ester of phthalic anhydride and 2-hydroxyethyl(meth)acrylate, or vinylnaphthalene sulfonic acid.
A second example of the water-soluble or hydrophilic polymerizable substance in practical use is a (meth)acrylic ester of a polyhydric alcohol provided with hydrophilicity by a polyethylene oxide chain such as tetraethylene glycol di(meth)acrylate, which is an industrially produced compound having two or more polymerizable functional groups.
Another example thereof is a (meth)acrylic ester of a hydrophilic polyepoxide derived from a polyhydric alcohol such as glycerin, disclosed in Japanese Patent Application Laid-Open No. 2000-117960.
Examples of a water-soluble or hydrophilic polymerization initiator include: a hydroxyphenyl ketone polymerization initiator provided with water-solubility through addition of a polyethylene oxide chain, disclosed in Japanese Patent Application Laid-Open No. 2000-186242; and a thioxanthone polymerization initiator provided with water-solubility through addition of a polyethylene oxide chain or an anionic functional group, disclosed in Japanese Patent Application Laid-Open No. 2002-1879 18. Further, Japanese Patent Application Laid-Open No. S57-197289 discloses an invention relating to an acylphosphine oxide polymerization initiator having an ionic group as a polymerization initiator useful for polymerization of an aqueous polymerizable composition such as a coating material, a printing ink, an unsaturated polyester molding material, or a recording material.
Further, Japanese Patent Application Laid-Open No. 2000-159621 discloses an invention in which the water-soluble acylphosphine oxide polymerization initiator described in Japanese Patent Application Laid-Open No. S57-197289 is used as a polymerizable composition for dental use.
Active energy ray curable aqueous ink has excellent properties as described above, but degradation of its curing performance through dilution with water is pointed out in some cases. The curing performance may be improved by increasing contents of a polymerizable substance and polymerization initiator. However, when active energy ray curable aqueous ink is applied to an inkjet system, the ink is subjected to limitation in viscosity or the like. As a result, the curing performance greatly varies depending on properties of individual materials.
Active energy ray curable aqueous ink preferably applied to the inkjet system must include a water-soluble polymerizable substance which is formed of a material having low viscosity and favorable flowability applicable to high density nozzles, which can be added to the ink in a large amount, and which provides excellent hardness of an ink film or excellent adhesion of the ink film on to a recording medium after curing. The active energy ray curable aqueous ink preferably applied thereto must further include a water-soluble polymerization initiator which highly responds to irradiation of active energy rays in a short period of time, which can generate large amounts of radicals, and which provides excellent initiation efficiency.
The polymerizable substance in the first example has one polymerizable functional group in a molecule, and has a low polymerization rate and a very low degree of crosslinking of a cured product. Thus, the polymerization substance cannot serve as a main material for the aqueous polymerizable composition.
The polymerizable substance of the second example is not water-soluble if a length of an ethylene oxide chain is short. In contrast, if the length of the ethylene oxide chain is long, the polymerizable substance is water-soluble, but has problems in that physical properties of a polymerized or cured solid are often insufficient in performance such as hardness and adhesiveness, required for paint or ink.
The polymerizable substance disclosed in Japanese Patent Application Laid-Open No. 2000-117960 is highly water-soluble, has a high polymerization rate by active energy rays, and has excellent physical properties of a cured product. However, the polymerizable substance has such a feature that it has numerous hydroxyl groups and thus has a slightly high viscosity. Thus, the polymerizable substance may be added to ink in a small amount.
The polymerization initiator disclosed in Japanese Patent Application Laid-Open No. 2000-186242 is liquid, has excellent compatibility with water and a polymerizable substance, but provides slightly insufficient initiation efficiency. The polymerization initiator disclosed in Japanese Patent Application Laid-Open No. 2002-187918 is the type of hydrogen abstraction and requires a hydrogen donor such as tertiary amine. Further, the polymerization initiator is colored and is not suitable for clear ink. The polymerization initiator disclosed in Japanese Patent Application Laid-Open No. S57-197289 has features of high quantum yield of radical generation, high reactivity of radicals having unpaired electrons on a phosphorus atom, and long actual life of generated radicals. Thus, the polymerization initiator has high initiation efficiency and favorable water-solubility.
The water-soluble polymerization initiator disclosed in Japanese Patent Application Laid-Open No. S57-197289 has excellent properties as described above, but an actual application thereof remains as the dental material described in Japanese Patent Application Laid-Open No. 2000-159621 or the like. In this application, an irradiation time of the active energy rays is not particularly limited, and thus the initiation efficiency is not a required item.
High initiation efficiency is effective in applications requiring high curing rate of printing ink or the like. Such application presumably has not been tried because of insufficient reactivity of a water-soluble polymerizable substance combined with a polymerization initiator.
In contrast, performance of the entire aqueous polymerizable composition may significantly degrade because of insufficient initiation efficiency of a water-soluble polymerization initiator combined with a water-soluble polymerizable substance having high reactivity.
As described above, the inventors of the present invention have found for the first time that a combination of two factors including reactivity of a water-soluble polymerizable substance and initiation efficiency of a water-soluble polymerization initiator must be focused for ensuring sufficient curing performance as an active energy ray curable aqueous ink composition and obtaining adaptability to a wide range of applications.